As a powering device of a power steering to assist the steering by operation of the steering wheel by the driver by an external power, there is known an electric power steering device using an electric motor. Since the electric power steering device using an electric motor can freely control the magnitude of the steering assist force independently of the steering angle or the steering angular velocity, it is tried in various manners to control the operation of the electric power steering device in relation with the operating conditions of the vehicle. As one of them it is proposed in Japanese Patent Laid-open Publication Hei 8-104249 to incorporate a reaction component based upon a detected value of the behavior of the vehicle including the yaw rate in the driving instructions to the electric motor, while detecting the driving force of the steered wheels, and to modify the parameter gain of the reaction component based upon the detected driving force.